This invention relates to a magnesia ceramic for insulation of high-frequency electricity producible by relatively low temperature burning, excelling in high-frequency dielectric loss and almost totally unsusceptible to warping or cracking.
Sintered magnesia has a particularly low dielectric loss at high frequency and, in this sense, is an excellent material for high-frequency insulators. On the other hand, since magnesia has an extremely high melting point of 2800.degree. C, it is difficult to sinter magnesia of high purity sintered completely. Moreover, sintered magnesia has a disadvantage that it abounds in hydrating property. Because of these disadvantages, magnesia has scarcely been used as the material for high-frequency insulators. It is known in the art that, generally by adding kaolin, clay and a metal oxide to magnesia, desired sintering of magnesia is rendered easier to accomplish and at the same time the sintered magnesia is rendered more resistant to the hydration. Incorporation of such additives, however, entails various problems such as degradation of electric properties at high frequency, for example.
Formerly the inventors made a discovery that a magnesia ceramic formed by low-temperature sintering and possessed of resistance to water and excellent insulation from high-frequency electricity is obtained by treating powdered magnesia in ethanol containing phosphoric acid for thereby giving rise to magnesium phosphate in the green body. In the case of this ceramic, however, there is involved the disadvantage that the ceramic's dielectric loss is relatively high and the ceramic, in the shape of a thin plate in which the ceramic is most frequently used as the insulator for electric current, tends to warp or crack.
The inventors continued devoted study in search of a way of overcoming this disadvantage and have consequently acquired a knowledge that the disadvantage is overcome by incorporating calcium-magnesium-phosphate at specific proportions into the green body prepared for the ceramic for thereby inhibiting formation of free magnesium phosphate, Mg.sub.3 (PO.sub.4).sub.2, or beta calcium phosphate, .beta.-Ca.sub.3 (PO.sub.4).sub.2, and at the same time adding thereto a metal oxide capable of preventing the high-frequency dielectric properties from degradation. The present invention has been perfected on the basis of this knowledge.
The major object of this invention is to provide a magnesia ceramic for electric insulation, which excels in resistance to hydration and in high-frequency dielectric loss and which, in the shape of a thin plate, is almost totally free of warping or cracking.